Amplifier circuits are widely used in electronic systems to allow processing of weak signals. Typically, an amplifier circuit will produce substantially constant gain only over a specific frequency range referred to as the bandwidth of the amplifier. In some applications the operability of an amplifier circuit is limited by this bandwidth. Consequently, much design effort is focused on increasing the bandwidth of amplifier circuits. Amplifier circuits are also limited by their phase-stability. The amplifier may introduce a phase difference between the input signal and the output signal. Each of these limitations may reduce the usefulness of an amplifier.
An exemplary system that requires a high bandwidth phase-stable amplifier is a phase-only direction finding circuit. Such circuits may be used, for example, in various military applications such as, ARM seekers, radar warning and homing receivers, ELS receivers, as well as radar, and the like. Typically, such electronic circuits incorporate an IF amplifier. To produce useful information, the IF amplifier should provide low amplitude modulation-to-phase modulation (phase stability), as well as channel-to-channel gain/phase tracking over a wide bandwidth.
Heretofore known phase-only direction finding systems typically use limiter circuits, successive detection log amplifier circuits, and true log amplifier circuits. Each of these circuits includes some form of an amplifier circuit. Successive detection log amplifiers typically comprise a limiter in each stage to limit the inputs to succeeding stages. This reduces "overdrive" and helps speed up the overload recovery time. True log amplifiers also use a limiter. In a true log amplifier, a limiter is typically placed in parallel with a unity gain stage. The limiter is typically used for small signal amplification and the unity gain stage typically is used to handle signals up to the largest expected signal without limiting.
Use of a limiter in each of these circuits creates a common problem known as phase runout or phase instability. Substantial phase runout may create false readings in a phase-only direction finding system. In true log IF amplifiers, phase runout may be caused by the size of the IF input signal. For example, the phase of the output IF signal may change as the input signal gets larger and larger because the small signal insertion phase of the limiter portion is not the same as the insertion phase of the unity gain portion. At small signals, the phase of the output is that of the limiter portion. At large signals, the phase of the output is that of the unity gain portion. Another contributor to the phase runout is that the insertion phase changes in the limiter portion as the limiter is driven into harder and harder limiting. This may cause a shift of more than several degrees in the amplifier. In successive detection log amplifiers, the limiters may also cause a phase runout problem if the phase is used for monopulse direction finding. Successive detection log amplifiers have had a problem with not phase tracking for unequal inputs since one channel may have one or more additional stages in limiting relative to the other channel.
Another problem with heretofore known phase-only direction finding systems is that the bandwidth of the system may not be adequate to detect the possible frequency range of incoming signals. The bandwidth of the system is typically limited by the bandwidth of the amplifier circuit.